This invention relates generally to thermal barrier coatings for superalloy substrates and in particular to a multilayer, ceramic thermal barrier coating having low thermal conductivity for superalloy blades and vanes in gas turbine engines.
As gas turbine engine technology advances and engines are required to be more efficient, gas temperatures within the engines continue to rise. However, the ability to operate at these increasing temperatures is limited by the ability of the superalloy turbine blades and vanes to maintain their mechanical strength when exposed to the heat, oxidation, and corrosive effects of the impinging gas. One approach to this problem has been to apply a protective thermal barrier coating which insulates the blades and vanes and inhibits oxidation and hot gas corrosion.
Typically, thermal barrier coatings are applied to a superalloy substrate and include a bond coat and a ceramic top layer. The ceramic top layer is applied either by the process of plasma spraying or by the process of electron beam physical vapor deposition (EB-PVD). Use of the EB-PVD process results in the outer ceramic layer having a columnar grained microstructure. Gaps between the individual columns allow the columnar grains to expand and contract without developing stresses that could cause spalling. Strangman, U.S. Pat. Nos. 4,321,311, 4,401,697, and 4,405,659 disclose thermal barrier coatings for superalloy substrates that contain a MCrAlY layer, an alumina layer, and an outer columnar grained ceramic layer. A more cost effective system is disclosed in Strangman U.S. Pat. No. 5,514,482 which teaches a thermal barrier coating for a superalloy substrate that contains an aluminide layer, an alumina layer, and an outer columnar grained ceramic layer.
The ceramic layer is commonly zirconia stabilized with yttria. The prior art teaches that the amount of yttria can range from 6 percent to 35 percent of the layer. (see U.S. Pat. Nos. 5,238,752 and 4,321,310). It is also known in the prior art that cubic zirconia, which is zirconia stabilized with 20 percent yttria, has a significantly lower thermal conductivity relative to tetragonal zirconia which is stabilized with 6 to 8 percent yttria. However, despite the disadvantage of higher thermal conductivity most commercially available thermal barrier coatings use tetragonal zirconia stabilized with 7 percent yttria for the ceramic layer because it is more reliable due to its superior capability to resist spalling and particulate erosion.
Accordingly, there is a need for a thermal barrier coating having a ceramic layer that has thermal conductivity less than or equal to that of cubic zirconia and resistance to spalling of tetragonal zirconia as well as a need for a method to make such a coating.
An object of the present invention is to provide a superalloy article having a ceramic layer that has thermal conductivity less than or equal to that of cubic zirconia and resistance to spalling of tetragonal zirconia.
Another object of the present invention is to provide a thermal barrier coating system having a ceramic layer that has thermal conductivity less than or equal to that of cubic zirconia and resistance to spalling of the tetragonal zirconia.
Yet another object of the present invention is to provide an improved electron beam-physical vapor deposition process for making such ceramic layers.
Yet still another object of the present invention is to provide a chamber for use in such improved electron beam-physical vapor deposition process.
The present invention achieves these objects by providing a thermal barrier coating that includes an aluminide or MCrAlY bond coat and a columnar ceramic layer applied to the bond coat by electron beam-physical vapor deposition. The ceramic coat is comprised of a plurality of layers of cubic zirconia stabilized with 20 percent yttria with the interfaces between layers decorated with particles selected from a group of second phase metal oxides such as Ta2O5 and alumina. The preferred concentration of Ta2O5 and/or alumina particles within the yttria stabilized zirconia is 1 to 4 weight %. An alternative is to codeposit the second phase metal oxide with the stabilized zirconia. Though not essential to the invention a layer of tetragonal zirconia stabilized with 7% yttria may be deposited both under and over the ceramic coat.
An improved electron beam-physical vapor deposition process is also disclosed. This method includes the step of mounting in a chamber a component(s) to be coated, an ingot of cubic zirconia and an ingot of Ta2O5. The two ingots being angularly spaced apart and preferably separated by a baffle. The ends of each of the ingots are bombarded with a stream of electrons to form vapors of each. The component is then alternatingly exposed to vapor deposition from the two vapor streams by rotation of the surfaces to be coated.